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RE: [EnergyPlus_Support] Exterior Radiative Heat Transfer Coefficients
Almost. These are indeed linearized coefficients for calculating the radiative
heat transfer components, however, they each use a different delta T. The sky
temperature is used with the "Rad to Sky" coefficient, outdoor dry bulb is used
as an asumption for the ground temperature with the "Rad to Ground"
coefficient, and the "Rad to Air" coefficient also uses outdoor dry bulb unless
Other Side Coefficients are being used.
I presume that you saw the (rather brief) definitions of these variables in the
Input Output Reference (p. 128, pdf p. 158).
In the Engineering Reference, the equations which use these variables are shown
in the section "Heat Balance Cases" beginning on page 53 (pdf p. 91). These
variables translate as follows:
Surface Ext Rad To Air Coeff = HA = HExtSurf (which includes both convective HO
and radiative HA)
Surface Ext Rad To Sky Coeff = HS = HSky
Surface Ext Rad To Ground Coeff = HG = HGround
The algorithm samples are a bit out of date, because the current code keeps HO
and HA separate now instead of combining them first.
Hope this helps. We'll make a note to try to cross-reference these better and
update any inconsistencies with current code.
Mike
On 2 Dec 2004, at 10:33, Griffith, Brent wrote:
>
> Peter,
>
> I think you have it right. These are linearized radiation coefficients that are
> comparable to convection coefficients. For example the coefficient for sky is
> calculated using,
>
> h_rad = Sigma * Emittance * View_Factor * ((TSurf**4)-(TSky**4))/(TSurf-TSky)
>
> Brent
>
>
> -----Original Message-----
> From: Peter G. Loutzenhiser [mailto:post@xxxxxxxxxxx]
> Sent: Thursday, December 02, 2004 10:03 AM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] Exterior Radiative Heat Transfer Coefficients
>
>
> Hello all,
>
> I am looking some of the inputs for output variables in EnergyPlus and have come
> across three outputs that I am not exactly certain what they mean. They include
> 1) Surface Ext Rad to Air Coeff, 2) Surface Ext Rad to Sky Coeff, and 3) Surface
> Ext Ground Coeff. I have looked at the Engineering Manuel which explains the
> calculation method thoroughly,but not the coefficients. Are these coefficients
> used to calculate the impact of radiation in a form similar to the exterior
> convective heat transfer coefficient? Is it reasonable to define the radiative
> component of heat transfer as the sum of these values and make the following
> calculation?:
>
> qrad=h_rad*surface_area(T_outside_temp-T_exterior_surface_temp)
>
> Or are the coefficients defined in a different way?
>
> Thank you so much for your help.
>
> Peter
>
>
>
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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